Gear shifting on target speed reduction in vehicle speed control system

ABSTRACT

A vehicle speed control system for a vehicle with an engine and an automatic transmission is comprised of a coast switch for decreasing a set vehicle speed and a controller connected with the coast switch. The controller controls a vehicle speed at the set vehicle speed by controlling the engine and the automatic transmission, and maintains a gear ratio of the automatic transmission at the gear ratio set at the moment before decreasing the set vehicle speed when the coast switch is being operated to decrease the set vehicle speed. Therefore, even if the throttle opening is opened to accelerate the vehicle, the engine rotation speed is never radically increased under such a transmission condition. This prevents the engine from generating noises excessively.

TECHNICAL FIELD

The present invention relates to a vehicle speed control system forcontrolling a vehicle speed, and more particularly to a vehicle speedcontrol system which controls a vehicle so as to automatically cruisethe vehicle at a set vehicle speed.

BACKGROUND ART

A Japanese Patent Provisional Publication No. (Heisei) 1-60437 disclosesa vehicle speed control system which is arranged to decelerates avehicle by a shift down control of a transmission in addition to athrottle control of an engine when a coast switch for lowering a setspeed is switched on.

DISCLOSURE OF INVENTION

However, when the coast switch is continuously and excessively switchedon such that the set speed is excessively lowered than an aimed setspeed, it is necessary to increase the lowered set speed to the aimedset speed by switching on an accelerate switch. For example, when it wasdesired to lower the set speed from 80 km/h to 60 km/h but when the setspeed was excessively lowered to 50 km/h by pressing the coast switchsix times (80 km/h−6×5 km/h), it is necessary to press the accelerateswitch twice to return the set speed to 60 km/h. Since the shift-downtransmission control is started in reply to the lowering operation ofthe set speed, the transmission connected to the conventional vehiclespeed control system has already executed a shift down operation at themoment when the set speed is increased. Accordingly, in such asituation, the vehicle speed control system outputs an accelerationcommand to the controlled system. As a result, the vehicle in theshift-down condition is accelerated by increasing the throttle opening.This operation will excessively increase the engine rotation speed andexcessively generate noises.

It is therefore an object of the present invention to provide animproved vehicle speed control system which solves the above-mentionedproblem.

A vehicle speed control system according to the present invention is fora vehicle equipped with an engine and an automatic transmission, andcomprises a coast switch for decreasing a set vehicle speed and acontroller connected with the coast switch. The controller is arrangedto control a vehicle speed at the set vehicle speed by controlling athrottle of the engine and the automatic transmission, and to maintain agear ratio of the automatic transmission at the gear ratio set at themoment before decreasing the set vehicle speed when the coast switch isbeing operated to decrease the set vehicle speed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a structure of a vehicle speed controlsystem according to the present invention.

FIG. 2 is a block diagram showing a structure of a lateral-accelerationvehicle-speed correction-quantity calculating block 580.

FIG. 3 is a graph showing a relationship between a vehicle speedV_(A)(t) and a cutoff frequency fc of a low pass filter.

FIG. 4 is a graph showing a relationship between a correctioncoefficient CC for calculating a vehicle speed correction quantityV_(SUB)(t) and a value Y_(G)(t) of the lateral acceleration.

FIG. 5 is a graph showing a relationship between a natural frequencyω_(nSTR) and the vehicle speed.

FIG. 6 is a graph showing a relationship between an absolute value of adeviation between vehicle speed V_(A)(t) and a maximum value V_(SMAX) ofa command vehicle speed, and a command vehicle speed variationΔV_(COM)(t).

FIG. 7 is a block diagram showing a structure of a command drive torquecalculating block 530.

FIG. 8 is a map showing an engine nonlinear stationary characteristic.

FIG. 9 is a map showing an estimated throttle opening.

FIG. 10 is a map showing a shift map of a CVT.

FIG. 11 is a map showing an engine performance.

FIG. 12 is a block diagram showing another structure of command drivetorque calculating block 530.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIGS. 1 to 12, there is shown a vehicle speed controlsystem according to an embodiment of the present invention.

FIG. 1 shows a block diagram showing a construction of the vehicle speedcontrol system according to the embodiment of the present invention.With reference to FIGS. 1 to 12, the construction and operation of thevehicle speed control system according to the present invention will bediscussed hereinafter.

The vehicle speed control system according to the present invention isequipped on a vehicle and is put in a standby mode in a manner that avehicle occupant manually switches on a system switch (not shown) of thespeed control system. Under this standby mode, when a set switch 20 isswitched on, the speed control system starts operations.

The vehicle speed control system comprises a vehicle speed control block500 which is constituted by a microcomputer and peripheral devices.Blocks in vehicle speed control block 500 represent operations executedby this microcomputer. Vehicle speed control block 500 receives signalsfrom a steer angle sensor 100, a vehicle speed sensor 10, the set switch20, a coast switch 30, an accelerate (ACC) switch 40, an engine speedsensor 80, an accelerator pedal sensor 90 and a continuously variabletransmission (CVT) 70. According to the signals received, vehicle speedcontrol block 500 calculates various command values and outputs thesecommand values to CVT 70, a brake actuator 50 and a throttle actuator 60of the vehicle, respectively, to control an actual vehicle speed at atarget vehicle speed.

A command vehicle speed determining block 510 of vehicle speed controlblock 500 calculates a command vehicle speed V_(COM)(t) by each controlcycle, such as by 10 ms. A suffix (t) denotes that the value with thesuffix (t) is a valve at the time t and is varied in time series (timeelapse). In some graphs, such suffix (t) is facilitated.

A command vehicle speed maximum value setting block 520 sets a vehiclespeed V_(A)(t) as a command vehicle speed maximum value V_(SMAX) (targetspeed) at time when set switch 30 is switched on. Vehicle speed V_(A)(t)is an actual vehicle speed which is detected from a rotation speed of atire rotation speed by means of a vehicle speed sensor 10.

After command vehicle speed maximum value V_(SMAX) is set by theoperation of set switch 20, command vehicle speed setting block 520decreases command vehicle speed maximum value V_(SMAX) by 5 km/h inreply to one push of coast switch 30. That is, when coast switch 30 ispushed a number n of times (n times), command vehicle speed V_(SMAX) isdecreased by n×5 km/h. Further, when coast switch 30 has been pushed fora time period T (sec.), command vehicle speed V_(SMAX) is decreased by avalue T/1(sec.)×5 km/h.

Similarly, after command vehicle speed maximum value V_(SMAX) is set bythe operation of set switch 20, command vehicle speed setting block 520increases command vehicle speed maximum value V_(SMAX) by 5 km/h inreply to one push of ACC switch 40. That is, when ACC switch 40 ispushed a number n of times (n times), command vehicle speed maximumvalue V_(SMAX) is increased by n×5 km/h. Further, ACC switch 40 has beenpushed for a time period T (sec.), command vehicle speed maximum valueV_(SMAX) is increased by a value T/1(sec.)×5 (km/h).

A lateral acceleration (lateral G) vehicle-speed correction-quantitycalculating block 580 receives a steer angle θ(t) from steerangle-sensor 100 and vehicle speed V_(A)(t) from vehicle speed sensor10, and calculates a vehicle speed correction quantity V_(SUB)(t) whichis employed to correct the command vehicle speed V_(COM)(t) according toa lateral acceleration (hereinafter, it called a lateral-G). Morespecifically, lateral-G vehicle-speed correction-quantity calculatingsection 580 comprises a steer angle signal low-pass filter (hereinafter,it called a steer angle signal LPF block) 581, a lateral-G calculatingblock 582 and a vehicle speed correction quantity calculation map 583,as shown in FIG. 2.

Steer angle signal LPF block 581 receives vehicle speed V_(A)(t) andsteer angle θ(t) and calculates a steer angle LPF value θ_(LPF)(t).Steer angle LPF value θ_(LPF)(t) is represented by the followingequation (1).θ_(LPF)(t)=θ(t)/(TSTR·s+1)  (1)In this equation (1), s is a differential operator, and TSTR is a timeconstant of the low-pass filter (LPF) and is represented byTSTR=1/(2π·fc). Further, fc is a cutoff frequency of LPF and isdetermined according to vehicle speed V_(A)(t) as shown by a map showinga relationship between cutoff frequency fc and vehicle speed V_(A)(t) inFIG. 3. As is clear from the map of FIG. 3, cutoff frequency fc becomessmaller as the vehicle speed becomes higher. For example, a cutofffrequency at the vehicle speed 100 km/h is smaller than that at thevehicle speed 50 km/h.

Lateral-G calculating block 582 receives steer angle LPF valueθ_(LPF)(t) and vehicle speed V_(A)(t) and calculates the lateral-GY_(G)(t) from the following equation (2).Y _(G)(t)={V _(A)(t)²·θ_(LPF)(t)}/{N·W·[1+A·V _(A)(t)²]}  (2)In this equation (2), W is a wheelbase dimension of the vehicle, N is asteering gear ratio, and A is a stability factor. The equation (2) isemployed in case that the lateral G of the vehicle is obtained from thesteer angle.

When the lateral G is obtained by using a yaw-rate sensor and processingthe yaw rate ψ(t) by means of a low-pass filter (LPF), the lateral-GY_(G)(t) is obtained from the following equations (3) and (4).Y _(G)(t)=V _(A)(t)·ψ_(LPF)  (3)ψ_(LPF)=ψ(t)/(T _(YAW) ·s+1)  (4)In the equation (4), T_(YAW)is a time constant of the low-pass filter.The time constant T_(YAW) increases as vehicle speed V_(A)(t) increases.

Vehicle speed correction calculation map 583 calculates a vehicle speedcorrection quantity V_(SUB)(t) which is employed to correct commandvehicle speed V_(COM)(t) according to lateral-G Y_(G)(t). Vehicle speedcorrection quantity V_(SUB)(t) is calculated by multiplying a correctioncoefficient CC determined from the lateral G and a predeterminedvariation limit of command vehicle speed V_(COM)(t). In this embodiment,the predetermined variation limit of command vehicle speed V_(COM)(t) isset at 0.021 (km/h/10 ms)=0.06 G. The predetermined variation limit ofthe command vehicle speed is equal to the maximum value of a variation(corresponding to acceleration/deceleration) ΔV_(COM)(t) of the commandvehicle speed shown in FIG. 6.V _(SUB)(t)=CC×0.021(km/h/10 ms)  (5)

As mentioned later, the vehicle speed correction quantity V_(SUB)(t) isadded as a subtraction term in the calculation process of the commandvehicle speed V_(COM)(t) which is employed to control the vehicle speed.Accordingly, command vehicle speed V_(COM)(t) is limited to a smallervalue as vehicle correction quantity V_(SUB)(t) becomes larger.

Correction coefficient CC becomes larger as lateral-G Y_(G) becomeslarger, as shown in FIG. 4. The reason thereof is that the change ofcommand vehicle speed V_(COM)(t) is limited more as the lateral-Gbecomes larger. However, when the lateral-G is smaller than or equal to0.1 G as shown in FIG. 4, correction coefficient CC is set at zero sinceit is decided that it is not necessary to correct command vehicle speedV_(COM)(t). Further, when the lateral-G is greater than or equal to 0.3G, correction coefficient CC is set at a predetermined constant value.That is, the lateral-G never becomes greater than or equal to 0.3 G asfar as the vehicle is operated under a usual driving condition.Therefore, in order to prevent the correction coefficient CC from beingset at an excessively large value when the detection value of thelateral-G erroneously becomes large, the correction coefficient CC isset at such a constant value, such as at 2.

When a driver requests to increase the target vehicle speed by operatingaccelerate switch 40, that is, when acceleration of the vehicle isrequested, the command vehicle speed V_(COM)(t) is calculated by addingpresent vehicle speed V_(A)(t) and command vehicle speed variationΔV_(COM)(t) and by subtracting vehicle speed correction quantityV_(SUB)(t) from the sum of present vehicle speed V_(A)(t) and commandvehicle speed variation ΔV_(COM)(t).

Therefore, when command vehicle speed variation ΔV_(COM)(t) is greaterthan vehicle speed correction quantity V_(SUB)(t), the vehicle isaccelerated. When command vehicle speed variation ΔV_(COM)(t) is smallerthan vehicle speed correction quantity V_(SUB)(t), the vehicle isdecelerated. Vehicle speed correction quantity V_(SUB)(t) is obtained bymultiplying the limit value of the command vehicle speed variation (amaximum value of the command vehicle speed variation) with correctioncoefficient CC shown in FIG. 4. Therefore, when the limit value of thecommand vehicle speed variation is equal to the command vehicle speedvariation and when correction coefficient CC is 1, the amount foracceleration becomes equal to the amount for deceleration. In case ofFIG. 4, when Y_(G)(t)=0.2, the amount for acceleration becomes equal tothe amount for deceleration. Accordingly, the present vehicle speed ismaintained when the correction coefficient CC is 1. In this example,when the lateral-G Y_(G)(t) is smaller than 0.2, the vehicle isaccelerated. When the lateral-G Y_(G)(t) is larger than 0.2, the vehicleis decelerated.

When the driver requests to lower the target vehicle speed by operatingcoast switch 30, that is, when the deceleration of the vehicle isrequested, the command-vehicle speed V_(COM)(t) is calculated bysubtracting command vehicle speed variation ΔV_(COM)(t) and vehiclespeed correction quantity V_(SUB)(t) from present vehicle speedV_(A)(t). Therefore, in this case, the vehicle is always decelerated.The degree of the deceleration becomes larger as vehicle speedcorrection quantity V_(SUB)(t) becomes larger. That is, vehicle speedcorrection quantity V_(SUB)(t) increases according to the increase ofthe lateral-G Y_(G)(t). The above-mentioned value 0.021 (km/h/10 ms) hasbeen defined on the assumption that the vehicle is traveling on ahighway.

As mentioned above, vehicle speed correction quantity V_(SUB)(t) isobtained from the multiple between the correction coefficient CCaccording to the lateral acceleration and the limit value of the commandvehicle speed variation V_(COM)(t). Accordingly, the subtract term(vehicle speed correction quantity) increases according to the increaseof the lateral acceleration so that the vehicle speed is controlled soas to suppress the lateral-G. However, as mentioned in the explanationof steer angle signal LPF block 581, the cutoff frequency fc is loweredas the vehicle speed becomes larger. Therefore the time constant TSTR ofthe LPF is increased, and the steer angle LPF θ_(LPF)(t) is decreased.Accordingly, the lateral acceleration estimated at the lateral-Gcalculating block 581 is also decreased. As a result, the vehicle speedcorrection quantity V_(SUB)(t), which is obtained from the vehicle speedcorrection quantity calculation map 583, is decreased. Consequently, thesteer angle becomes ineffective as to the correction of the commandvehicle speed. In other words, the correction toward the decrease of theacceleration becomes smaller due to the decrease of vehicle speedcorrection quantity V_(SUB)(t).

More specifically, the characteristic of the natural frequency ω_(nSTR)relative to the steer angle is represented by the following equation(6).ω_(nSTR)=(2W/V _(A))√{square root over ([Kf·Kr·(1+A·V _(A) ²)/m _(V)·I])}  (6)In this equation (6), Kf is a cornering power of one front tire, Kr is acornering power of one rear tire, W is a wheelbase dimension, m_(v) is avehicle weight, A is a stability factor, and I is a vehicle yaw inertiamoment.

The characteristic of the natural frequency ω_(nSTR) performs such thatthe natural frequency ω_(nSTR) becomes smaller and the vehicleresponsibility relative to the steer angle degrades as the vehicle speedincreases, and that the natural frequency ω_(nSTR) becomes greater andthe vehicle responsibility relative to the steer angle is improved asthe vehicle speed decreases. That is, the lateral-G tends to begenerated according to a steering operation as the vehicle speed becomeslower, and the lateral-G due to the steering operation tends to besuppressed as the vehicle speed becomes higher. Therefore, the vehiclespeed control system according to the present invention is arranged tolower the responsibility by decreasing the cutoff frequency fc accordingto the increase of the vehicle speed so that the command vehicle speedtends not to be affected by the correction due to the steer angle as thevehicle speed becomes higher.

A command vehicle speed variation determining block 590 receives vehiclespeed V_(A)(t) and command vehicle speed maximum value V_(SMAX) andcalculates the command vehicle speed variation ΔV_(COM)(t) from the mapshown in FIG. 6 on the basis of an absolute value |V_(A)−V_(SMAX)| of adeviation between the vehicle speed V_(A)(t) and the command vehiclespeed maximum value V_(SMAX).

The map for determining command vehicle speed variation ΔV_(COM)(t) isarranged as shown in FIG. 6. More specifically, when absolute value|V_(A)−V_(SMAX)| of the deviation is within a range B in FIG. 6, thevehicle is quickly accelerated or decelerated by increasing commandvehicle speed variation ΔV_(COM)(t) as the absolute value of thedeviation between vehicle speed V_(A)(t) and command vehicle speedmaximum value V_(SMAX) is increased within a range where command vehiclespeed variation ΔV_(COM)(t) is smaller than acceleration limit α fordeciding the stop of the vehicle speed control. Further, when theabsolute value of the deviation is small within the range B in FIG. 6,command vehicle speed variation ΔV_(COM)(t) is decreased as the absolutevalue of the deviation decreases within a range where the driver canfeel an acceleration of the vehicle and the command vehicle speedvariation ΔV_(COM)(t) does not overshoot maximum value V_(SMAX) of thecommand vehicle speed. When the absolute value of the deviation is largeand within a range A in FIG. 6, command vehicle speed variationΔV_(COM)(t) is set at a constant value which is smaller thanacceleration limit α, such as at 0.06 G. When the absolute value of thedeviation is small and within a range C in FIG. 6, command vehicle speedvariation ΔV_(COM)(t) is set at a constant value, such as at 0.03 G.

Command vehicle speed variation determining block 590 monitors vehiclespeed correction quantity V_(SUB)(t) outputted from lateral-G vehiclespeed correction quantity calculating block 580, and decides that atraveling on a curved road is terminated when vehicle speed correctionquantity V_(SUB)(t) is returned to zero after vehicle speed correctionquantity V_(SUB)(t) took a value except for zero from zero. Further,command vehicle speed variation determining block 590 detects whethervehicle speed V_(A)(t) becomes equal to maximum value V_(SMAX) of thecommand vehicle speed.

When it is decided that the traveling on a curved road is terminated,the command vehicle speed variation ΔV_(COM)(t) is calculated fromvehicle speed V_(A)(t) at the moment when it is decided that thetraveling on a curved road is terminated, instead of determining thecommand vehicle speed variation ΔV_(COM)(t) by using the map of FIG. 6on the basis of the absolute value of a deviation between vehicle speedV_(A)(t) and maximum value V_(SMAX) of the command vehicle speed. Thecharacteristic employed for calculating the command vehicle speedvariation ΔV_(COM)(t) under the curve-traveling terminated conditionperforms a tendency which is similar to that of FIG. 6. Morespecifically, in this characteristic employed in this curve terminatedcondition, a horizontal axis denotes vehicle speed V_(A)(t) instead ofabsolute value |V_(A)(t)−V_(SMAX)|. Accordingly, command vehicle speedvariation ΔV_(COM)(t) becomes small as vehicle speed V_(A)(t) becomessmall. This processing is terminated when vehicle speed V_(A)(t) becomesequal to maximum value V_(SMAX) of the command vehicle speed.

Instead of the above determination method of command vehicle speedvariation ΔV_(COM)(t) at the termination of the curved road traveling,when vehicle speed correction quantity V_(SUB)(t) takes a value exceptfor zero, it is decided that the curved road traveling is started. Underthis situation, vehicle speed V_(A)(t1) at a moment t1 of starting thecurved road traveling may be previously stored, and command vehiclespeed variation ΔV_(COM)(t) may be determined from a magnitude of adifference ΔV_(A) between vehicle speed V_(A)(t1) at the moment t1 ofthe start of the curved road traveling and vehicle speed V_(A)(t2) atthe moment t2 of the termination of the curved road traveling. Thecharacteristic employed for calculating the command vehicle speedvariation ΔV_(COM)(t) under this condition performs a tendency which isopposite to that of FIG. 6. More specifically, in this characteristiccurve, there is employed a map in which a horizontal axis denotesvehicle speed V_(A)(t) instead of |V_(A)(t)−V_(SMAX)|. Accordingly,command vehicle speed variation ΔV_(COM)(t) becomes smaller as vehiclespeed V_(A)(t) becomes larger. This processing is terminated whenvehicle speed V_(A)(t) becomes equal to maximum value V_(SMAX) of thecommand vehicle speed.

That is, when the vehicle travels on a curved road, the command vehiclespeed is corrected so that the lateral-G is suppressed within apredetermined range. Therefore, the vehicle speed is lowered in thissituation generally. After the traveling on a curved road is terminatedand the vehicle speed is decreased, the command vehicle speed variationΔV_(COM)(t) is varied according to vehicle speed V_(A)(t) at the momentof termination of the curved road traveling or according to themagnitude of the difference ΔV_(A) between vehicle speed V_(A)(t1) atthe moment t1 of staring of the curved road traveling and vehicle speedV_(A)(t2) at the moment t2 of the termination of the curved roadtraveling.

Further, when the vehicle speed during the curved road traveling issmall or when vehicle speed difference ΔV_(A) is small, command vehiclespeed variation ΔV_(COM)(t) is set small and therefore the accelerationfor the vehicle speed control due to the command vehicle speed isdecreased. This operation functions to preventing a large accelerationfrom being generated by each curve when the vehicle travels on a windingroad having continuous curves such as a S-shape curved road. Similarly,when the vehicle speed is high at the moment of the termination of thecurved road traveling, or when vehicle speed difference ΔV_(A) is small,it is decided that the traveling curve is single and command vehiclespeed variation ΔV_(COM)(t) is set at a large value. Accordingly, thevehicle is accelerated just after the traveling of a single curved roadis terminated, and therefore the driver of the vehicle becomes free froma strange feeling due to the slow-down of the acceleration.

Command vehicle speed determining block 510 receives vehicle speedV_(A)(t), vehicle speed correction quantity V_(SUB)(t), command vehiclespeed variation ΔV_(COM)(t) and maximum value V_(SMAX) of the commandvehicle speed and calculates command vehicle speed V_(COM)(t) asfollows.

(a) When maximum value V_(SMAX) of the command vehicle speed is greaterthan vehicle speed V_(A)(t), that is, when the driver requestsaccelerating the vehicle by operating accelerate switch 40 (or a resumeswitch), command vehicle speed V_(COM)(t) is calculated from thefollowing equation (7).

 V _(COM)(t)=min[V _(SMAX) , V _(A)(t)+ΔV _(COM)(t)−V _(SUB)(t)]  (7)

That is, smaller one of maximum value V_(SMAX) and the value[V_(A)(t)+ΔV_(COM)(t)−V_(SUB)(t)] is selected as command vehicle speedV_(COM)(t).

(b) When V_(SMAX)=V_(A)(t), that is, when the vehicle travels at aconstant speed, command vehicle speed V_(COM)(t) is calculated from thefollowing equation (8).V _(COM)(t)=V _(SMAX) −V _(SUB)(t)  (8)That is, command vehicle speed V_(COM)(t) is obtained by subtractingvehicle speed correction quantity V_(SUB)(t) from maximum value V_(SMAX)of the command vehicle speed.

(c) When maximum value V_(SMAX) of the command vehicle speed is smallerthan vehicle speed V_(A)(t), that is, when the driver requests todecelerate the vehicle by operating coast switch 30, command vehiclespeed V_(COM)(t) is calculated from the following equation (9).V _(COM)(t)=max[V _(SMAX) , V _(A)(t)−ΔV _(COM)(t)−V _(SUB)(t)]  (9)That is, larger one of maximum value V_(SMAX) and the value[V_(A)(t)−ΔV_(COM)(t)−V_(SUB)(t)] is selected as command vehicle speedV_(COM)(t).

Command vehicle speed V_(COM)(t) is determined from the above-mentionedmanner, and the vehicle speed control system controls vehicle speedV_(A)(t) according to the determined command vehicle speed V_(COM)(t).

A command drive torque calculating block 530 of vehicle speed controlblock 500 in FIG. 1 receives command vehicle speed V_(COM)(t) andvehicle speed V_(A)(t) and calculates a command drive torque d_(FC)(t).FIG. 7 shows a construction of command drive torque calculating block530.

When the input is command vehicle speed V_(COM)(t) and the output isvehicle speed V_(A)(t), a transfer characteristic (function) G_(V)(s)thereof is represented by the following equation (10).G _(V)(s)=1/(T _(V) ·s+1)·e ^((−Lv·s))  (10)In this equation (10), T_(V) is a first-order lag time constant, andL_(V) is a dead time due to a delay of a power train system.

By modeling a vehicle model of a controlled system in a manner oftreating command drive torque d_(FC)(t) as a control input (manipulatedvalue) and vehicle speed V_(A)(t) as a controlled value, the behavior ofa vehicle power train is represented by a simplified linear model shownby the following equation (11).V _(A)(t)=1/(m _(V) ·Rt·s)·e ^((−Lv·s)) ·d _(FC)(t)  (11)In this equation (11), Rt is an effective radius of a tire, and m_(V) isa vehicle mass (weight).

The vehicle model, which employs command drive torque d_(FC)(t) as aninput and vehicle speed V_(A)(t) as an output, performs an integralcharacteristic since the equation (11) of the vehicle model is of a 1/stype.

Although the controlled system (vehicle) performs a non-linearcharacteristic which includes a dead time L_(V) due to the delay of thepower train system and varies the dead time L_(V) according to theemployed actuators and engine, the vehicle model, which employs thecommand drive torque d_(FC)(t) as an input and vehicle speed V_(A)(t) asan output, can be represented by the equation (11) by means of theapproximate zeroing method employing a disturbance estimator.

By corresponding the response characteristic of the controlled system ofemploying the command drive torque d_(FC)(t) as an input and vehiclespeed V_(A)(t) as an output to a characteristic of the transfer functionG_(V)(s) having a predetermined first-order lag T_(V) and the dead timeL_(V), the following relationship is obtained by using C₁(s), C₂(s) andC₃(s) shown in FIG. 7.C ₁(s)=e ^((−Lv·s))/(T _(H) ·s+1)  (12)C ₂(s)=(m _(V) ·Rt·s)/(T _(H) ·s+1)  (13)d _(V)(t)=C ₂(s)·V_(A)(t)−C₁(s)·d _(FC)(t)  (14)In these equations (12), (13) and (14), C₁(s) and C₂(s) are disturbanceestimators for the approximate zeroing method and perform as acompensator for suppressing the influence due to the disturbance and themodeling.

When a norm model G_(V)(s) is treated as a first-order low-pass filterhaving a time constant T_(V) upon neglecting the dead time of thecontrolled system, the model matching compensator C₃(s) takes a constantas follows.C ₃(t)=m_(V) ·Rt/T _(V)  (15)

From these compensators C₁(s), C₂(s) and C₃(s), the command drive torqued_(FC)(t) is calculated from the following equation (16)d _(FC)(t)=C ₃(s)·{V _(COM)(t)−V _(A)(t)}−{C ₂(s)·V _(A)(t)−C ₁(s)·d_(FC)(t)}  (16)

A drive torque of the vehicle is controlled on the basis of commanddrive torque d_(FC)(t). More specifically, the command throttle openingis calculated so as to bring actual drive torque d_(FA)(t) closer tocommand drive torque d_(FC)(t) by using a map indicative of an enginenon-linear stationary characteristic. This map is shown in FIG. 8, therelationship represented by this map has been previously measured andstored. Further, when the required toque is negative and is not ensuredby the negative drive torque of the engine, the vehicle control systemoperates the transmission and the brake system to ensure the requirednegative torque. Thus, by controlling the throttle opening, thetransmission and the brake system, it becomes possible to modify theengine non-linear stationary characteristic into a linearizedcharacteristic.

Since CVT 70 employed in this embodiment according to the presentinvention is provided with a torque converter with a lockup mechanism,vehicle speed control block 500 receives a lockup signal LUs from acontroller of CVT 70. The lockup signal LUs indicates the lockupcondition of CVT 70. When vehicle speed control block 500 decides thatCVT 70 is put in an un-lockup condition on the basis of the lockupsignal LU_(S), vehicle speed control block 500 increases the timeconstant T_(H) employed to represent the compensators C₁(s) and C₂(s) asshown in FIG. 7. The increase of the time constant T_(H) decreases thevehicle speed control feedback correction quantity, which corresponds toa correction coefficient for keeping a desired response characteristic.Therefore, it becomes possible to adjust the model characteristic to theresponse characteristic of the controlled system under the un-lockupcondition, although the response characteristic of the controlled systemunder the un-lockup condition delays as compared with that of thecontrolled system under the lockup condition. Accordingly, the stabilityof the vehicle speed control system is ensured under both lockupcondition and un-lockup condition.

Command drive torque calculating block 530 shown in FIG. 7 isconstructed by compensators C₁(s) and C₂(s) for compensating thetransfer characteristic of the controlled system and compensator C₃(s)for achieving a response characteristic previously designed by adesigner.

Further, command drive torque calculating block 530 may be constructedby a pre-compensator C_(F)(s) for compensating so as to ensure a desiredresponse characteristic determined by the designer, a norm modelcalculating block C_(R)(s) for calculating the desired responsecharacteristic determined by the designer and a feedback compensator C₃(s)′ for compensating a drift quantity (a difference between the targetvehicle speed and the actual vehicle speed) with respect to the responsecharacteristic of the norm model calculating section C_(R)(s), as shownin FIG. 12.

The pre-compensator C_(F)(s) calculates a standard command drive torqued_(FC1)(t) by using a filter represented by the following equation (17),in order to achieve the transfer function G_(V)(s) of the actual vehiclespeed V_(A)(t) with respect to the command vehicle speed V_(COM)(t).d _(FC1)(t)=m _(V) ·R _(T) ·s·V _(COM)(t)/(T _(V) ·s+1)  (17)

Norm model calculating block C_(R)(s) calculates a target responseV_(T)(t) of the vehicle speed control system from the transfer functionG_(V)(s) and the command vehicle speed V_(COM)(t) as follows.V _(T)(t)=G _(V)(s)·V _(COM)(t)  (18)

Feedback compensator C₃(s)′ calculates a correction quantity of thecommand drive torque so as to cancel a deviation thereby when thedeviation between the target response V_(T)(t) and the actual vehiclespeed V_(A)(t) is caused. That is, the correction quantity d_(V)(t)′ iscalculated from the following equation (19).d _(V)(t)′=[(K _(P) ′·s+K _(I)′)/s][V _(T)(t)−V _(A)(t)]  (19)In this equation (19), K_(P) is a proportion control gain of thefeedback compensator C₃(s)′, K_(I) is an integral control gain of thefeedback compensator C₃(s)′, and the correction quantity d_(V)(t)′ ofthe drive torque corresponds to an estimated disturbance d_(V)(t) inFIG. 7.

When it is decided that CVT 70 is put in the un-lockup condition fromthe lockup condition signal LUs, the correction quantity d_(V)(t)′ iscalculated from the following equation (20).d _(V)(t)′=[(K _(P) ′·s+K _(I)′)/s][V _(T)(t)−V _(A)(t)]  (20)In this equation (20), K_(P)′>K_(P), and K_(I)′>K_(I). Therefore, thefeedback gain in the un-lockup condition of CVT 70 is decreased ascompared with that in the-lockup condition of CVT 70. Further, commanddrive torque d_(FC)(t) is calculated from a standard command drivetorque d_(FC)(t) and the correction quantity d_(V)(t)′ as follows.d _(FC)(t)=d _(FC1)(t)+d_(V)(t)′  (21)

That is, when CVT 70 is put in the un-lockup condition, the feedbackgain is set at a smaller value as compared with the feedback gain in thelockup condition. Accordingly, the changing rate of the correctionquantity of the command drive torque becomes smaller, and therefore itbecomes possible to adapt the response characteristic of the controlledsystem which characteristic delays under the un-lockup condition of CVT70 as compared with the characteristic in the lockup condition.Consequently, the stability of the vehicle speed control system isensured under both of the lockup condition and the un-lockup condition.

Next, the actuator drive system of FIG. 1 will be discussed hereinafter.

A command gear ratio calculating block 540 of vehicle speed controlblock 500 in FIG. 1 receives command drive torque d_(FC)(t), vehiclespeed V_(A)(t), the output of coast switch 30 and the output ofaccelerator pedal sensor 90. Command gear ratio calculating block 540calculates a command gear ratio DRATIO(t), which is a ratio of an inputrotation speed and an output rotation speed of CVT 70, on the basis ofthe received information and outputs command gear ratio DRATIO(t) to CVT70 as mentioned hereinafter.

(a) When coast switch 30 is put in an off state, an estimated throttleopening TVO_(ESTI) is calculated from the throttle opening estimationmap shown in FIG. 9 on the basis of vehicle speed V_(A)(t) and commanddrive torque d_(FC)(t). Then, a command engine rotation speed N_(IN-COM)is calculated from the CVT shifting map shown in FIG. 10 on the basis ofestimated throttle opening TVO_(ESTI) and vehicle speed V_(A)(t).Further, command gear ratio DRATIO(t) is obtained from the followingequation (22) on the basis of vehicle speed V_(A)(t) and command enginerotation speed N_(IN-COM).DRATIO(t)=N _(IN-COM)·2π·Rt/[60·V _(A)(t)·Gf]  (22)In this equation (22), Gf is a final gear ratio.

(b) When coast switch 30 is put in an on state, that is, when maximumvalue V_(SMAX) of the command vehicle speed is decreased by switching oncoast switch 30, the previous value DRATIO(t−1) of command gear ratio ismaintained as the present command gear ratio DRATIO(t). Therefore, evenwhen coast switch 30 is continuously switched on, command gear ratioDRATIO(t) is maintained at the value set just before the switching on ofcoast switch 30 until coast switch is switched off. That is, the shiftdown is prohibited for a period from the switching on of coast switch 30to the switching off of coast switch 30.

More specifically, when the set speed of the vehicle speed controlsystem is once decreased by operating coast switch 30 and is thenincreased by operating accelerate switch 40, the shift down isprohibited during this period. Therefore, even if the throttle openingis opened to accelerate the vehicle, the engine rotation speed is neverradically increased under such a transmission condition. This preventsthe engine from generating noises excessively.

An actual gear ratio calculating block 550 of FIG. 1 calculates anactual gear ratio RATIO(t), which is a ratio of an actual input rotationspeed and an actual output rotation speed of CVT 70, from the followingequation on the basis of the engine rotation speed N_(E)(t) and vehiclespeed V_(A)(t) which is obtained by detecting an engine spark signalthrough engine speed sensor 80.RATIO(t)=N _(E)(t)/[V _(A)(t)·Gf·2π·Rt]  (23)

A command engine torque calculating block 560 of FIG. 1 calculates acommand engine torque TE_(COM)(t) from command drive torque d_(FC)(t),actual gear ratio RATIO(t) and the following equation (24).TE _(COM)(t)=d _(FC)(t)/[Gf·RATIO(t)]  (24)

A target throttle opening calculating block 570 of FIG. 1 calculates atarget throttle opening TVO_(COM) from the engine performance map shownin FIG. 11 on the basis of command engine torque TE_(COM)(t) and enginerotation speed N_(E)(t), and outputs the calculated target throttleopening TVO_(COM) to throttle actuator 60.

A command brake pressure calculating block 630 of FIG. 1 calculates anengine brake torque TE_(COM)′ during a throttle full closed conditionfrom the engine performance map shown in FIG. 11 on the basis of enginerotation speed N_(E)(t). Further, command brake pressure calculatingblock 630 calculates a command brake pressure REF_(PBRK)(t) from thethrottle full-close engine brake torque TE_(COM)′, command engine torqueTE_(COM)(t) and the following equation (25).REF _(PBRK)(t)=(TE _(COM) −TE _(COM)′)·Gm·Gf/{4·(2·AB·RB·μB)}  (25)In this equation (25), Gm is a gear ratio of CVT 70, AB is a wheelcylinder force (cylinder pressure×area), RB is an effective radius of adisc rotor, and μB is a pad friction coefficient.

Next, the suspending process of the vehicle speed control will bediscussed hereinafter.

A vehicle speed control suspension deciding block 620 of FIG. 1 receivesan accelerator control input APO detected by accelerator pedal sensor 90and compares accelerator control input APO with a predetermined value.The predetermined value is an accelerator control input APO₁corresponding to a target throttle opening TVO_(COM) inputted from atarget throttle opening calculating block 570, that is a throttleopening corresponding to the vehicle speed automatically controlled atthis moment. When accelerator control input APO is greater than apredetermined value, that is, when a throttle opening becomes greaterthan a throttle opening controlled by throttle actuator 60 due to theaccelerator pedal depressing operation of the drive, vehicle speedcontrol suspension deciding block 620 outputs a vehicle speed controlsuspending signal.

Command drive torque calculating block 530 and target throttle openingcalculating block 570 initialize the calculations, respectively in replyto the vehicle speed control suspending signal, and the transmissioncontroller of CVT 70 switches the shift-map from a constant speedtraveling shift-map to a normal traveling shift map. That is, thevehicle speed control system according to the present invention suspendsthe constant speed traveling, and starts the normal traveling accordingto the accelerator pedal operation of the driver.

The transmission controller of CVT 70 has stored the normal travelingshift map and the constant speed traveling shift map, and when thevehicle speed control system according to the present invention decidesto suspend the constant vehicle speed control, the vehicle speed controlsystem commands the transmission controller of CVT 70 to switch theshift map from the constant speed traveling shift map to the normaltraveling shift map. The normal traveling shift map has a highresponsibility characteristic so that the shift down is quickly executedduring the acceleration. The constant speed traveling shift map has amild characteristic which impresses a smooth and mild feeling to adriver when the shift map is switched from the constant speed travelingmode to the normal traveling mode.

Vehicle speed control suspension deciding block 620 stops outputting thevehicle speed control suspending signal when the accelerator controlinput APO returns to a value smaller than the predetermined value.Further, when the accelerator control input APO is smaller than thepredetermined value and when vehicle speed V_(A)(t) is greater than themaximum value V_(SMAX) of the command vehicle speed, vehicle speedcontrol suspension deciding block 620 outputs the deceleration commandto the command drive torque calculating block 530.

When the output of the vehicle speed control suspending signal isstopped and when the deceleration command is outputted, command drivetorque calculating block 530 basically executes the deceleration controlaccording to the throttle opening calculated at target throttle openingcalculating block 570 so as to achieve command drive torque d_(FC)(t).However, when command drive torque d_(FC)(t) cannot be achieved only byfully closing the throttle, the transmission control is further employedin addition to the throttle control. More specifically, in such a largedeceleration force required condition, command gear ratio calculatingblock 540 outputs the command gear ratio DRATIO (shift down command)regardless the road gradient, such as traveling on a down slope or aflat road. CVT 70 executes the shift down control according to thecommand gear ratio DRATIO to supply the shortage of the deceleratingforce.

In addition to the above arrangement of employing the shift down controlof CVT 70 based on the magnitude of the deceleration in the restartingoperation of the vehicle speed control, the shift down control may beutilized when a time period to the target vehicle speed, which isachieved by the full closing of the throttle, becomes greater than apredetermined time period. More specifically, vehicle speed controlblock 500 may be arranged to employ the shift down control of the CVT inorder to decelerate the vehicle at the target vehicle speed when thepredetermined time period cannot be ensured by fully closing thethrottle.

Further, when command drive torque d_(FC)(t) is not ensured by both thethrottle control and the transmission control, and when the vehicletravels on a flat road, the shortage of command drive torque d_(FC)(t)is supplied by employing the brake system. However, when the vehicletravels on a down slope, the braking control by the brake system isprohibited by outputting a brake control prohibiting signal BP fromcommand drive torque calculating block 530 to a command brake pressurecalculating block 630. The reason for prohibiting the braking control ofthe brake system on the down slope is as follows.

If the vehicle on the down slope is decelerated by means of the brakesystem, it is necessary to continuously execute the braking. Thiscontinuous braking may cause the brake fade. Therefore, in order toprevent the brake fade, the vehicle speed control system according tothe present invention is arranged to execute the deceleration of thevehicle by means of the throttle control and the transmission controlwithout employing the brake system when the vehicle travels on a downslope.

With the thus arranged suspending method, even when the constant vehiclespeed cruise control is restarted after the constant vehicle speedcruise control is suspended in response to the temporal accelerationcaused by depressing the accelerator pedal, a larger deceleration ascompared with that only by the throttle control is ensured by the downshift of the transmission. Therefore, the conversion time period to thetarget vehicle speed is further shortened. Further, by employing acontinuously variable transmission (CVT 70) for the deceleration, ashift shock is prevented even when the vehicle travels on the downslope. Further, since the deceleration ensured by the transmissioncontrol and the throttle control is larger than that only by thethrottle control and since the transmission control and the throttlecontrol are executed to smoothly achieve the drive torque on the basisof the command vehicle speed variation ΔV_(COM), it is possible tosmoothly decelerate the vehicle while keeping the deceleration degree atthe predetermined value. In contrast to this, if a normal non-CVTautomatic transmission is employed, a shift shock is generated duringthe shift down, and therefore even when the larger deceleration isrequested, the conventional system employed a non-CVT transmission hasexecuted only the throttle control and has not executed the shift downcontrol of the transmission.

By employing a continuously variable transmission (CVT) with the vehiclespeed control system, it becomes possible to smoothly shift down thegear ratio of the transmission. Therefore, when the vehicle isdecelerated for continuing the vehicle speed control, a decelerationgreater than that only by the throttle control is smoothly executed.

Next, a stopping process of the vehicle speed control will be discussed.

A drive wheel acceleration calculating block 600 of FIG. 1 receivesvehicle speed V_(A)(t) and calculates a drive wheel accelerationα_(OBS)(t) from the following equation (26).α_(OBS)(t)=[K _(OBS) ·s/(T _(OBS)·s² +s+K _(OBS))]·V_(A)(t)  (26)In this equation (26), K_(OBS) is a constant, and T_(OBS) is a timeconstant.

Since vehicle speed V_(A)(t) is a value calculated from the rotationspeed of a tire (drive wheel), the value of vehicle speed V_(A)(t)corresponds to the rotation speed of the drive wheel. Accordingly, drivewheel acceleration α_(OBS)(t) is a variation (drive wheel acceleration)of the vehicle speed obtained from the derive wheel speed V_(A)(t).

Vehicle speed control stop deciding block 610 compares drive wheelacceleration α_(OBS)(t) calculated at drive torque calculating block 600with the predetermined acceleration limit α which corresponds to thevariation of the vehicle speed, such as 0.2 G. When drive wheelacceleration α_(OBS)(t) becomes greater than the acceleration limit α,vehicle speed control stop deciding block 610 outputs the Vehicle speedcontrol stopping signal to command drive torque calculating block 530and target throttle opening calculating block 570. In reply to thevehicle speed control stopping signal, command drive torque calculatingblock 530 and target throttle opening calculating block 570 initializethe calculations thereof respectively. Further, when the vehicle speedcontrol is once stopped, the vehicle speed control is not started untilset switch 20 is again switched on.

Since the vehicle speed control system shown in FIG. 1 controls thevehicle speed at the command vehicle speed based on command vehiclespeed variation ΔV_(COM) determined at command vehicle speed variationdetermining block 590. Therefore, when the vehicle is normallycontrolled, the vehicle speed variation never becomes greater than thelimit of the command vehicle speed variation, for example, 0.06 G=0.021(km/h/10 ms). Accordingly, when drive wheel acceleration α_(OBS)(t)becomes greater than the predetermined acceleration limit a whichcorresponds to the limit of the command vehicle speed acceleration,there is a possibility that the drive wheels are slipping. That is, bycomparing drive wheel acceleration α_(OBS)(t) with the predeterminedacceleration limit α, it is possible to detect the generation ofslippage of the vehicle. Accordingly, it becomes possible to execute theslip decision and the stop decision of the vehicle speed control, byobtaining drive wheel acceleration α_(OBS)(t) from the output of thenormal vehicle speed sensor without providing an acceleration sensor ina slip suppressing system such as TCS (traction control system) andwithout detecting a difference between a rotation speed of the drivewheel and a rotation speed of a driven wheel. Further, by increasing thecommand vehicle speed variation ΔV_(COM), it is possible to improve theresponsibility of the system to the target vehicle speed.

Although the embodiment according to the present invention has beenshown and described such that the stop decision of the vehicle speedcontrol is executed on the basis of the comparison between the drivewheel acceleration α_(OBS)(t) and the predetermined value, the inventionis not limited to this and may be arranged such that the stop decisionis made when a difference between the command vehicle variation ΔV_(COM)and drive wheel acceleration α_(OBS)(t) becomes greater than apredetermined value.

Command vehicle speed determining block 510 of FIG. 1 decides whetherV_(SMAX)<V_(A), that is, whether the command vehicle speed V_(COM)(t) isgreater than vehicle speed V_(A)(t) and is varied to the deceleratingdirection. Command vehicle speed determining block 510 sets commandvehicle speed V_(COM)(t) at vehicle speed V_(A)(t) or a predeterminedvehicle speed smaller than vehicle speed V_(A)(t), such as at a valueobtained by subtracting 5 km/h from vehicle speed V_(A)(t), and sets theinitial values of integrators C₂(s) and C₁(s) at vehicle speed V_(A)(t)so as to set the output of the equationC₂(s)·V_(A)(t)−C₁(s)·d_(FC)(t)=d_(V)(t) at zero. As a result of thissettings, the outputs of C₁(s) and C₂(s) become V_(A)(t) and thereforethe estimated disturbance d_(V)(t) becomes zero. Further, this controlis executed when the variation ΔV_(COM) which is a changing rate ofcommand vehicle speed V_(COM) is greater in the deceleration directionthan the predetermined deceleration, such as 0.06G. With thisarrangement, it becomes possible to facilitate unnecessaryinitialization of the command vehicle speed (V_(A)(t)→V_(COM)(t)) andinitialization of the integrators, and to decrease the shock due to thedeceleration.

Further, when the command vehicle speed (command control value at eachtime until the actual vehicle speed reaches the target vehicle speed) isgreater than the actual vehicle speed and when the time variation(change rate) of the command vehicle speed is turned to the deceleratingdirection, by changing the command vehicle speed to the actual vehiclespeed or the predetermined speed smaller than the actual vehicle speed,the actual vehicle speed is quickly converged into the target vehiclespeed. Furthermore, it is possible to keep the continuing performance ofthe control by initializing the calculation of command drive torquecalculating block 530 from employing the actual vehicle speed or a speedsmaller than the actual vehicle speed.

Further, if the vehicle speed control system is arranged to execute acontrol for bringing an actual inter-vehicle distance closer to a targetinter-vehicle distance so as to execute a vehicle traveling whilekeeping a target inter-vehicle distance set by a driver with respect toa preceding vehicle, the vehicle speed control system is arranged to setthe command vehicle speed so as to keep the target inter-vehicledistance. In this situation, when the actual inter-vehicle distance islower than a predetermined distance and when the command vehicle speedvariation ΔV_(COM) is greater than the predetermined value (0.06 G) inthe deceleration direction, the change (V_(A)→V_(COM)) of the commandvehicle speed V_(COM) and the initialization of command drive torquecalculating block 530 (particularly, integrator) are executed. With thisarrangement, it becomes possible to quickly converge the inter-vehicledistance to the target inter-vehicle distance. Accordingly, theexcessive approach to the preceding vehicle is prevented, and thecontinuity of the control is maintained. Further, the decrease of theunnecessary initialization (V_(A)(t)→V_(COM)(t) and initialization ofintegrators) decreases the generation of the shift down shock.

The entire contents of Japanese Patent Applications No. 2000-143581filed on May 16, 2000 in Japan are incorporated herein by reference.

Although the invention has been described above by reference to acertain embodiment of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiment described above will occur to those skilled in the art, inlight of the above teaching. The scope of the invention is defined withreference to the following claims.

INDUSTRIAL APPLICABILITY

A vehicle speed control system according to the present invention isapplicable to an automotive vehicle equipped with a continuouslyvariable transmission.

1. A vehicle speed control system for a vehicle, the vehicle equippedwith an engine and an automatic transmission, the vehicle control systemcomprising: a coast switch for decreasing a set vehicle speed; and acontroller connected with said coast switch, said controller configuredfor: controlling a vehicle speed at the set vehicle speed by controllinga throttle of the engine and the automatic transmission; and maintaininga gear ratio of the automatic transmission at a gear ratio set at amoment before decreasing the set vehicle speed during a time period whenthe coast switch is continuously and excessively switched on.
 2. Thevehicle speed control system according to claim 1, wherein thecontroller maintains the gear ratio during the time period when thecoast switch is continuously and excessively switched on independent ofa road condition.
 3. A vehicle speed control system for a vehicle, thevehicle equipped with an engine and an automatic transmission, thevehicle speed control system comprising: a vehicle cruise speed settingdevice for setting a set vehicle speed; and a controller connected withsaid vehicle cruise speed setting device, said controller configuredfor: controlling a vehicle speed at the set vehicle speed by controllinga throttle of the engine and the automatic transmission; and fixing agear ratio of the automatic transmission during a time period ofcontinuously and excessively executing a decreasing operation of the setvehicle speed by said vehicle cruise speed device.
 4. The vehicle speedcontrol system as claimed in claim 3, wherein said controller commandsthe automatic transmission to prohibit a shift down when the set vehiclespeed is being decreased by continuously and excessively switching on acoast switch.
 5. The vehicle speed control system as claimed in claim 3,wherein said controller commands the engine to start a decelerationcontrol when the set vehicle speed is being decreased by continuouslyand excessively switching on a coast switch.
 6. The vehicle speedcontrol system as claimed in claim 3, wherein said vehicle cruise speedsetting device comprises: a set switch for setting the set vehiclespeed; a coast switch for decreasing the set vehicle speed; and anaccelerate switch for increasing the set vehicle speed, wherein saidvehicle cruise speed setting device is manually operated by a vehicleoccupant.
 7. The vehicle speed control system as claimed in claim 3,wherein said controller controls the engine rotation speed within apredetermined limit when a vehicle speed at a moment of restarting thevehicle speed control is greater than or equal to the set vehicle speed.8. The vehicle speed control system as claimed in claim 3, wherein thetime period of continuously and excessively executing the decreasingoperation corresponds to a time period during a time period when a coastswitch for decreasing the set vehicle speed is continuously andexcessively switched on.
 9. The vehicle speed control system accordingto claim 3, wherein the controller fixes the gear ratio during the timeperiod of continuously and excessively executing a decreasing operationof the set vehicle speed independent of a road condition.
 10. A methodfor executing a vehicle speed control, comprising: controlling a vehiclespeed at a set vehicle speed by controlling an engine and an automatictransmission of the vehicle; detecting whether the set vehicle speed isbeing decreased; and maintaining a gear ratio of the automatictransmission at a gear ratio set at a moment before the set vehiclespeed is decreased, during a time period when the coast switch iscontinuously and excessively switched on.
 11. The method according toclaim 10, wherein step of maintaining the gear ratio during the timeperiod when the coast switch is continuously and excessively switched onis performed independent of a road condition.